Adhesive tapes coated with adhesives on one or both sides are normally wound up at the end of the production process to give a roll in the form of an Archimedean spiral. In order to prevent the pressure-sensitive adhesives (PSAs) coming into contact with one another in the case of double-sided adhesives tapes, or to prevent PSA sticking to the carrier in the case of single-sided adhesive tapes, the adhesive tapes prior to winding are applied to a liner material (also referred to as release material) which is wound up together with the adhesive tape. Liner materials, or release liners, of this kind are familiar to the skilled person. As well as for lining single-sided or double-sided adhesive tapes, liners are also employed for lining labels.
A liner (release paper, release film) is not part of an adhesive tape or label, but merely an auxiliary means in its production, storage or further processing by punching. Furthermore, in contrast to an adhesive-tape carrier, a liner is not firmly joined to a layer of adhesive.
Abhesive coating materials are in widespread use for the production of liners in the coating, in particular, of sheet materials such as papers or films, in order to reduce the adhesion tendency of adhering products relative to these surfaces.
When a double-sided adhesive tape furnished with a liner is unwound, it is normally adhered by its open—that is, liner-free—PSA side to a substrate. During this procedure, the other PSA side still adheres to the coated surface of the liner to a sufficient extent to allow handling of the adhesive tape.
However, the liner must be removable from the adhesive tape. The liner itself, or the removal of the liner, must not have a substantially detrimental effect on the bond strength of the PSA, for its subsequent use.
At the same time, the stability of the antiadhesive coating (also called release coating) on the liner, in other words the abhesiveness, over long time periods is important in order to ensure the function of this coating and also to ensure the properties of the PSA lined with the liner.
Crosslinkable silicone systems are frequently used for release coating. They include mixtures of crosslinking catalysts and what are called thermally curable condensation-crosslinking or addition-crosslinking polysiloxanes. For condensation-crosslinking silicone systems, tin compounds, such as dibutylin diacetate, are frequently present as crosslinking catalysts in the composition.
Silicone-based release coatings on an addition-crosslinking basis can be cured by hydrosilylation. These release systems typically comprise the following constituents: an alkenylated polydiorganosiloxane (in particular, linear polymers with terminal alkenyl groups), a polyorganohydrosiloxane crosslinking agent, and a hydrosilylation catalyst.
Catalysts which have become established for addition-crosslinking silicone systems (hydrosilylation catalysts) include, for example, platinum or compounds of platinum, such as the Karstedt catalyst [a Pt(0) complex], for example.
Moreover, it is also possible to use photoactive catalysts, known as photoinitiators, in combination with UV-curable, cationically crosslinking, epoxide- and/or vinyl ether-based siloxanes, or UV-curable, free-radically crosslinking siloxanes such as, for instance, acrylate-modified siloxanes. Also possible is the use of electron beam-curable silicone acrylates. Depending on their intended use, such systems may also include further additions such as stabilizers or flow control assistants.
Furthermore, there are various kinds of organopolysiloxane compositions known which crosslink by heating or irradiation. These include compositions of the kind described, for example, in DE 600 01 779 T2, which crosslink through addition reaction, namely through temperature treatment of a mixture of an organopolysiloxane having hydrogen atoms attached directly to the silicon atoms, and an organopolysiloxane having vinyl groups attached directly to the silicon atoms, in the presence of a hydrosilylation catalyst.
Photopolymerizable organopolysiloxane compositions can be used as well. These include, by way of example, compositions which are crosslinked through the reaction between organopolysiloxanes which have hydrocarbon radicals that are substituted by (meth)acrylate groups and are attached directly to the silicon atoms, and in the presence of a photosensitizer (see EP 0 168 713 B1 or DE 38 20 294 C1). Likewise possible for use are compositions in which the crosslinking reaction between organopolysiloxanes which have mercapto-group-substituted hydrocarbon attached directly to the silicon atoms, and organopolysiloxanes having vinyl groups attached directly to the silicon atoms, is induced in the presence of a photosensitizer. Compositions of this kind are described in U.S. Pat. No. 4,725,630 A1, for example.
When use is made of the organopolysiloxane compositions, as described in DE 33 16 166 C1, for example, which have hydrocarbon radicals substituted by epoxy groups and attached directly to the silicon atoms, the crosslinking reaction is induced by release of a catalytic amount of acid, which is obtained by photo-decomposition of added onium salt catalysts. Other organopolysiloxane compositions curable by a cationic mechanism are materials which have, for example, propenyloxysiloxane end groups.
Of the silicones identified, the greatest economic importance is possessed by the addition-crosslinking (hydrosilylation-curing) silicones. One unwanted property of these systems, however, is their sensitivity toward catalyst poisons, such as heavy metal compounds, sulfur compounds, and nitrogen compounds, for example (in this regard, cf. “Chemische Technik, Prozesse and Produkte” by R. Dittmeyer et al., volume 5, 5th edition, Wiley-VCH, Weinheim, Germany, 2005, section 6-5.3.2, pages 1142). It is generally the case that electron donors can be considered to be platinum poisons (A. Colas, Silicone Chemistry Overview, Technical Paper, Dow Corning). The result of the presence of catalyst poisons is that the crosslinking reaction between the different constituents of a silicone release varnish no longer takes place or takes place only to a low extent.
In the preparation of antiadhesive silicone coatings, therefore, the presence of catalyst poisons, particularly of platinum poisons, is stringently avoided.
The practical application of the stated antiadhesive silicone coatings, however, exhibits a number of disadvantages.
For instance, the level of the removal force of a PSA from a silicone release liner is adjusted typically through silicone-MQ resins (silicone-methylsilicone-rubber resins). Depending on the requirement for the product, a different level is required for the removal forces. This makes it necessary to use, and therefore also to stock, a plurality of release liners with differing MQ resin contents.
Furthermore, release liners often cannot be used immediately after production, since the properties of the liner have not yet achieved a constant level. This is evident in particular from measurements of the release forces. These forces generally fall in the first days and weeks after production, until they reach a constant level. The effect is called “post-curing” or aftercrosslinking. The time between production of the release liner and coating with PSA is referred to, correspondingly, as post-curing time, aftercrosslinking time or else maturation time. Depending on the requirement relating to the release liner, and on the PSA used on the release liner, the maturation time may amount to several weeks. Because of the required maturation time, the release liners produced have to be stored, giving rise to storage costs.
A further problem emerges from the occasional observation during storage of an increase in the removal forces in the case of laminates of release liners with PSAs. To the skilled person, this effect is known as “adhesive lock-up” or, specifically in respect of acrylates, as “acrylic lock-up”. In certain circumstances, this effect means that the release liner is difficult if not impossible to remove from the PSA subsequently, meaning that the product becomes unusable.
It is an object of the invention to prevent or at least reduce the disadvantages of the prior art. It is desirable more particularly                to be able to set different removal forces between an acrylate-based PSA, located on a liner furnished with a silicone coating, and that liner, without effecting the bond strength of the PSA,        to shorten the required maturation time prior to the use of the release liner, and/or        to prevent acrylic lock-up.        